The ability of the mammalian immune system to recognize pathogenic DNA, such as DNA from viruses, is essential for the onset of intrinsic and innate immune responses. This recognition is accomplished by specialized cellular proteins called DNA sensors, which bind foreign DNA and elicit the secretion of cytokines to alert neighboring cells and inhibit the spread of infection. Until recently, the sensing of foreign DNA was thought to occur only in subcellular compartments normally devoid of DNA. However, this long-standing belief failed to explain how the cell detects nuclear-replicating DNA viruses. Our lab's recent characterization of the first identified nuclear DNA sensor, IFI16, has helped to firmly establish the concept of nuclear sensing. We demonstrated that IFI16 functions to sense herpesviruses, including the important pathogens, human cytomegalovirus (HCMV) and herpes simplex virus type 1 (HSV-1) (Li et al. PNAS 2012; Li et al. Cell Host Microbe 2013). The discovery that sensing can occur in the nucleus opens a new direction for research in immunity that will increase our understanding of how balanced immune responses work to maintain a healthy system and how their misregulation leads to immune disorders, cancers, and virus-induced morbidity and mortality. However, a fundamental question that has yet to be answered is how the immune signal is propagated from the nucleus. We found that, while IFI16 remains nuclear during the early stages of HCMV and HSV-1 infections, the IFI16-mediated induction of antiviral cytokines requires the endoplasmic reticulum adapter protein STING-a hub for DNA sensing pathways. The requirements for eliciting an immune response from the nucleus and the mechanism of immune signal propagation to the cytoplasm remain elusive. Our proposal will address these important questions. First, we will characterize the mechanism by which IFI16 rapidly co-localizes with viral DNA in the nucleus. We have discovered that IFI16 is recruited to promyelocytic leukemia (PML) nuclear bodies (NBs) following infection. PML-NBs have been implicated in antiviral response and shown to localize to origins of replication of herpesviruses. Using a multidisciplinary approach, we will test our hypothesis that PML-NBs function as interaction centers where IFI16 concentrates its binding of viral DNA, setting the stage for immune signal initiation. Second, we will use molecular biology, biochemistry, optogenetics, and live cell imaging to define the IFI16 properties required for initiating nuclear immune signals. We hypothesize that signal initiation requires IFI16 oligomerization via its pyrin domain at PML-NBs. Third, we will determine how the nucleus-derived immune signal is transmitted to the cytoplasm. We discovered that IFI16 interacts with interferon-inducible IFIT proteins, which shuttle to the nucleus upon infection. We propose mechanistic studies to test our hypothesis that IFI16-dependent immune signaling is relayed by such interactions to the STING hub. Collectively, our results will characterize a newly discovered aspect of immunity, nuclear sensing, by defining the fundamental mechanisms required for immune signal propagation.